By utilizing the phenomenon of forcibly reversing the polarization of a ferroelectric, a periodically poled region (periodically poled structure) can be formed in the ferroelectric. The periodically poled region thus formed has been used, for example, in an optical frequency modulator using a surface acoustic wave, an optical wavelength conversion element using nonlinear polarization reversal, an optical polarizer using a reversal structure of a prism shape or a lens shape, and the like. Particularly, an optical wavelength conversion element with high conversion efficiency can be realized by periodically reversing the nonlinear polarization of a nonlinear optical substance. By carrying out the wavelength conversion of a semiconductor laser or the like using the foregoing optical wavelength conversion element, it is possible to realize a small-size short wavelength light source applicable in the fields of printing, optical information processing and optical application measurement control. Furthermore, by carrying out the wavelength conversion of the light emitted from a high-output laser (fiber laser or solid-state laser) in the order of watt using the foregoing optical wavelength conversion element, it is possible to obtain a short wavelength visible light (green, blue) in the order of watt or a high-output ultraviolet laser, thereby realizing a high-output short wavelength light source applicable to high-luminance display, processing, exposure, etc.
As a prospective method for realizing the generation of high-output CW visible light in the order of watt by a single pass conversion of a fundamental wave, a technique of generating a second harmonic using a periodically poled LiNbO3 (hereinafter, abbreviated as “PPLN”) is known. Such LiNbO3 (hereinafter, abbreviated as “LN”) has a large nonlinear optical constant, CW short wavelength light in the order of watt can be generated by a single pass. However, the use of LN has caused problems of making an output unstable and necessitating a high temperature operation due to influences such as optical damage and green induced infrared absorption (hereinafter, abbreviated as “GRIIRA”).
As a solution to the foregoing problem, it is known to generate visible short wavelength light by a single pass structure using periodically poled MgO:LiNbO3 (hereinafter, abbreviated as “PPMgLN”). Since MgO:LiNbO3 (hereinafter, abbreviated as “MgLN”) has a higher nonlinear optical constant and better optical damage resistance and transmission characteristic in a short wavelength range than LN, it is promising as a highly nonlinear material capable of realizing a CW output in the order of watt at room temperature by the single pass structure.
Various methods have been proposed to suppress a reduction in conversion efficiency by an element temperature distribution in a wavelength conversion element caused by the laser light incident on the wavelength conversion element, examples of which includes the method of providing linearly heating means is provided as disclosed in patent document 1, the method of adjusting the position of the wavelength conversion element according to a temperature distribution in an optical axis direction so that a temperature difference in a crystal falls within 0.1° C. as disclosed in patent document 2, the method of reducing the temperature distribution in a propagation direction in the wavelength conversion element by adopting means for cooling an incident surface and an output surface of the element separately from the means for adjusting the temperature of a central part of the element as disclosed in patent document 3. Patent document 4 discloses the structure wherein four Peltier devices are provided on the side surfaces facing one another to maintain the wavelength conversion efficiency to suppress the temperature distribution in the widthwise direction of the wavelength conversion element.
However, with MgLN expected as a material capable of realizing a CW output in the order of watt at room temperature, another phenomenon different from the generation of the element temperature distribution caused by optical damage, GRIIRA or laser incidence occurred at the time of a high output, whereby a new problem of making a harmonic output unstable or damaging the crystal occurred. As a result of our verification of causes of this, it was found out that heat was generated in the crystal due to ultraviolet induced harmonic absorption caused by the interaction of a fundamental wave and a harmonic wave and a harmonic output became unstable. Particularly, in a light source in which a sum frequency wave of a fundamental wave and a harmonic wave is generated during the high-output harmonic generation, it was found that heat generation by harmonic absorption was notable. Conventionally, such heat generation by the harmonic absorption and sum frequency wave has not been recognized.
Further, a wavelength conversion element wherein a period for the periodical polarization inversion structure is changed to increase the tolerance range for phase matching conditions of the wavelength conversion element has been proposed as disclosed in patent document 5.
Patent Document 1:
Japanese Unexamined Patent Publication No. H11-125800
Patent Document 2:
Japanese Unexamined Patent Publication No. 2003-140211
Patent Document 3:
Japanese Unexamined Patent Publication No. 2004-53781
Patent Document 4:
Japanese Unexamined Patent Publication No. H05-204011
Patent Document 5:
Japanese Unexamined Patent Publication No. 2000-321610